The following abbreviations may be found in the specification and/or the drawing figures, and are defined as follows:
3GPP third generation partnership project
ACK acknowledgement
D2D device-to-device
DCF distributed coordination function
eNodeB base station of an LTE system (evolved NodeB)
E-UTRAN evolved universal terrestrial radio access network
HARQ hybrid automatic repeat request
HeNB home eNodeB (sometimes termed a femto cell)
HF hyper-frame number
IEEE Institute of Electrical and Electronics Engineers
ISM industrial scientific medical
LTE long term evolution (evolved UTRAN)
MAC medium access control
MCS modulation and coding scheme
NACK negative acknowledgement
PCell primary cell
PCF point coordination function
PDCP packet data convergence protocol
RAT radio access technology
RLC radio link control
RRC radio resource control
SCell second cell
SDU service data unit
SN serial number
STA station
TB transport block
UE user equipment
WLAN wireless local area network
The volume of wireless data being communicated over the scarce radio spectrum licensed to the various cellular carriers has grown dramatically over the past several years as the public has increasingly adopted various smartphone technologies. As a consequence much research has gone into utilizing various radio spectrum which has not been subject to licensed use in the past. Significant examples of such license-exempt spectrum is the ISM band which has long been used for WLAN, Bluetooth and Bigbee, and also television white spaces which were once reserved for television broadcast but have not been fully exploited as digital television signals have increasingly been carried over cable. Such license-exempt frequency bands are also termed shared bands or shared spectrum.
Some of this recent research is directed to the model in which the user camped on a cellular band has some of its data traffic ‘offloaded’ to the license-exempt band. However the offloading is to occur, since the license-exempt band is subject to use by other users not under control of the cellular user's cellular base station (and perhaps not even known to that base station), the cellular user's utilization of the license-exempt radio resources must follow the same protocol which those non-cellular users also follow. This helps assure that the available license-exempt bandwidth is fairly distributed among the various users.
Direct D2D communication is one emerging technique which can relieve traffic on the licensed bands. In D2D communications, the user devices communicate directly with one another without their transmissions having to first pass through the cellular network. There are varied ways to implement this emerging technology but for the case in which the D2D communication is on the license-exempt frequencies it represents traffic which could have but didn't need to go over the licensed cellular spectrum. So for example a cellular UE which wants to engage in D2D communications can operate simultaneously as a WLAN non-AP STA and utilize IEEE 802.11 protocols to contend with other users for radio resources (time and frequency slots or channels) on the license exempt band.
In order to assure fairness among various users as mentioned above, most shared band utilization protocols have some time limits on usage before a device needs to contend again with others for a channel. This is to prevent one user device which wins a channel contention from occupying the license-exempt channel indefinitely. The above-mentioned cellular UE seeking to conduct D2D communications on the shared band is also subject to those time limits. For example, in WLAN (the IEEE 802.11 family of specifications) the channels are in the ISM band and so are license-exempt, so it is reasonable that noise and interference will always be present. Given these generally poor channel conditions (relative to cellular), WLAN transmissions are typically limited to a few dozen milliseconds.
The examples below use LTE as the cellular system using licensed spectrum and WLAN as the system using license-exempt spectrum, but these are exemplary only and not limiting to the broader teachings herein. FIG. 1 illustrates an eNodeB 22 (which may be implemented as a HeNB) exercising centralized control over D2D communications between a first UE1 20 and a second UE2 21. In this arrangement the eNodeB 22 informs control signaling either on the cellular band 40 or on the shared band 50, and the UEs 20, 21 transmit data directly either on the cellular band 41 or on the shared band 51. The first UE1 20 can contend and occupy the shared bands (for example, the first UE1 20 pretends to be a virtual WLAN device to contend the shared band). Whether on the shared band or on the cellular band, the D2D transmission 41, 51 would use the air interface of the cellular system (e.g., scheduling protocol, frame arrangement, HARQ processes, etc.).
A problem arises in that the time limits for usage of the license-exempt band may not allow for sufficient amount of data to be transferred given the contention protocol that must be negotiated to use that band. The inventors anticipate that the situation may routinely arise where some data is transferred on the license-exempt band, only to have further portions of the same substantive communication (for example a unicast or multicast transmission) having to be transmitted over the licensed cellular band, and possibly back and forth as the involved device(s) alternately win and lose contention for a channel on the license-exempt band. As will be seen, contention protocols for WLAN and other license-exempt systems are not optimized for splitting a coherent data transmission back and forth between licensed and license-exempt band. Neither are the cellular systems optimized for such back and forth frequency switches.
Specifically, while the cellular devices seeking to use the license-exempt band for traffic need to observe the WLAN contention rules to obtain a shared band channel, it is inevitable that they will sometimes need to switch back to the cellular band to transmit at least some of their data. In order to use the shared band as much as possible such frequency switches are likely to occur routinely. If the latency to perform such a frequency switch is too large this will limit how much the cellular devices will be able to utilize the shared band.
In the conventional cellular protocols for handover or for activating a component carrier (SCell) on the license-exempt spectrum, there is the additional problem of how to guarantee no loss of data. To fully exploit a transmission opportunity on the license-exempt band for sending data there will be no resources left for the receiving UE2 21 to send its HARQ reply for the last of the scheduled data, potentially leading to the sending UE1 20 missing a necessary re-transmission to the receiving UE2 21. If one solves this by having the data receiver UE2 21 feeding back its HARQ status indication to the sending UE1 20 via RRC or RLC signaling to guarantee the HARQ delivery, the latency issue is not resolved.
Exemplary embodiments of the invention detailed below address this frequency-switching problem.